Sample introduction method and system for atomic spectrometry

ABSTRACT

A method of introducing a sample into an atomic spectrometer utilizes a spray head including a vibratable mesh. A liquid sample is conducted to one face of the mesh and the mesh is vibrated to expel sample droplets from the other face of the mesh into the proximal end of a flow passage axially spaced from the mesh. Also, a low pressure gas as flowed into the proximal end of the flow passage to mix with the droplets to form an aerosol in the flow passage. The vibrating of the mesh is controlled to provide in the aerosol a selected total volume of monodisperse droplets while the flow of the carrier gas is independently controlled to provide a selected rate of flow of the aerosol along the flow passage thereby to optimize consumption of the sample. Apparatus for practicing the method is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Provisional Patent ApplicationSer. No. 61/477,453, filed Apr. 20, 2011, the contents of which areincorporated by reference herein.

FIELD OF THE INVENTION

The technology described herein generally relates to systems, methodsand devices for providing sample introduction for atomic spectrometry,and more particularly for use of a piezoelectric aerosol generator in asample introduction system for atomic spectrometry.

BACKGROUND INFORMATION

Atomic spectrometry is an analytical technique useful for determiningthe is elemental composition of a sample by its electromagnetic or massspectrum. Typically atomic spectrometry methods are distinguished by thetype of spectrometry used or the atomization source. Types of atomicspectrometry include optical and mass spectrometry. Optical spectrometrycan be further divided into absorption, emission and fluorescencespectrometry. Systems for atomic spectrometry include any of a varietyof atomization sources. Of atomization sources, flames are the mostcommon due to their low cost and their simplicity. Inductively-coupledplasmas (ICP) are recognized for their outstanding analyticalperformance and their versatility. To perform atomic spectrometryanalysis, the sample is vaporized and atomized. For atomic massspectrometry, a sample must also be ionized. Vaporization, atomization,and ionization are often, but not always, accomplished with a singlesource. For efficiency in this process a sample to be analyzed isintroduced into the source in droplet form. Pneumatic nebulizers arecurrently the most widely used sample introduction systems for ICP massspectrometry.

Pneumatic nebulizers produce droplets of varying sizes and require spraysample chambers to essentially prevent larger droplets from beingtransported to the atomizer. Thus, a transport efficiency of 20% isexpected, with roughly 80% of the sample being wasted. In addition, aperistaltic pump is typically required to deliver liquid to thenebulizer, and use of the pump results in the analytical precision ofthe measurement being tied to the liquid delivery rate of the pump.Further, the use of the pump tends also to introduce signalperturbations caused by the peristaltic pump pulsations.

SUMMARY OF THE INVENTION

Our improved sample introduction technique for use in atomicspectrometry systems utilizes a piezoelectric aerosol generator toproduce a liquid aerosol with droplets in a narrow desired size rangeand a mixing or collection chamber in which the droplets are blendedwith a carrier gas that is at a relatively low pressure and has adesired flow rate. A control sub-system allows a user to activelycontrol the volume of the aerosol introduced to the collection chamberand also independently control the flow rate of the carrier gas, toprovide for optimized consumption of the sample.

The sample introduction system and method provides relatively highsample transport efficiency, on the order of 80% or more, utilizingrelatively uniform distributions of droplets sized 10 microns or less.This is in contrast to the conventional sample introduction systems thatemploy pneumatic nebulizers, and which lose a high percentage of thesample to the elimination of overly large droplets in a spray samplechamber and/or to a lack of volume control. Further, the improved sampleintroduction system does not use a peristaltic pump, and may instead usepumps with lower pump rates, resulting in increased measurementprecision.

Our improved sample introduction technique, which allows for independentadjustment of aerosol generation characteristics and ICP properties,provides improved analytical precision and improved speed, while alsoreducing sample waste.

BRIEF DESCRIPTION OF THE DRAWINGS

The technology is further described in the detailed description thatfollows, by reference to the noted drawings by way of non-limitingillustrative embodiments, in which like reference numerals representsimilar parts throughout the drawings. As should be understood, however,the technology is not limited to the precise arrangements andinstrumentalities depicted in the drawings, in which:

FIG. 1 is a schematic view of a sample introduction system constructedin accordance with an example embodiment for use with an atomicspectrometry system;

FIGS. 2A to 2C are more detailed schematic views of the sampleintroduction system of FIG. 1;

FIG. 3 is a schematic view of a control sub-system for the sampleintroduction system of FIG. 1;

FIG. 4 is a schematic view of electronic drive waveforms relating to anexample embodiment;

FIG. 5 is a more detailed drawing of a perforated mesh utilized in thesystem of FIG. 1, and

FIG. 6 is a fragmentary schematic view of another example embodiment ofthe sample introduction system.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENTS

In the following description, for purposes of explanation and notlimitation, specific details are set forth, such as particular valves,adsorbents, sensors, heating devices, gases, materials, analytes,configurations, devices, ranges, temperatures, components, techniques,vessels, samples, and processes, etc. in order to provide a is thoroughunderstanding of the present invention.

However, it will be apparent to one skilled in the art that the presentinvention may be practiced in other embodiments that depart from thesespecific details. Detailed descriptions of well-known valves,adsorbents, sensors, heating devices, gases, materials, analytes,configurations, devices, ranges, temperatures, components, techniques,vessels, samples, and processes are omitted so as not to obscure thedescription of the present invention. As used in the description, theterms “top,” “bottom,” “above,” “below,” “over,” “under,” “above,”“beneath,” “on top,” “underneath,” “up,” “down,” “upper,” “lower,”“front,” “rear,” “back,” “forward” and “backward” refer to the objectsreferenced when in the orientation illustrated in the drawings, whichorientation is not necessary for achieving the objects of the invention.

The technology described herein relates to a sample introduction systemfor atomic spectrometry that utilizes piezoelectric aerosol generation.For ease of understanding, other component parts of atomic spectrometrysystem that operate in conventional manners are depicted in the drawingsas a functional block 100 labeled “Atomic Spectrometer.” The AtomicSpectrometer may be an IPC mass spectrometry system, an IPC opticemission system, and so forth.

Referring to FIG. 1, the sample introduction system 10 employs apiezoelectric aerosol generator (PAG) that consists of a fluid feedsub-system 12, a spray head assembly 14, and a mixing chamber 16, inthis example an argon collection system. The sample introduction systemfurther includes a programmable controller having a user input keypadand which is depicted in FIG. 1 as component parts 20 ₁ and 20 ₂, thatallows is independent user control of droplet generation and carrier gasfeed as will be described.

The PAG utilizes, in the spray head assembly 14, a spray head 142 (FIGS.2A and 2B) that may be similar to spray heads described in U.S. Pat.Nos. 7,316,067; 5,518,179; 5,838,350 and 6,113,001, incorporated byreference herein in their entireties. However, in the sampleintroduction system 10, the material utilized for a perforated meshcomponent 144 of the spray head 14 is selected based, at least in part,on the properties of the sample. In one example, the sample is an acidand the mesh is made of steel or of a plastic, such as Kapton® polyimidefilm. Also, the diameters and configuration of nozzles 146, formed asperforations in the mesh 144, are selected to produce a desired burstquantity and size range of droplets. Micrographs of a suitable steelmesh 144 containing nozzles 146 in the form of tiny perforations isdiscussed below in connection with FIG. 5.

Referring now to FIGS. 1 and 2A-C, sample fluid from a fluid source (notshown) is provided to a fluid feed cavity 124 through a fluid feed inlet122 and excess fluid is removed from the cavity through a fluid outlet126. The fluid feed cavity 124 is sealed against the rear of the sprayhead 142 and presents to the spray head 142 the sample that is to beformed into droplets. The spray head 142 includes the mesh 144 in alaminated assembly that also includes a piezoelectric transducer and asubstrate, which are together depicted in FIG. 2B by reference numeral148. The control sub-system component 20 ₁ connects to the transducerover lines 150 and provides a periodic drive waveform 400 (FIG. 4), suchas a square wave or sine wave, to actuate the transducer, which, inturn, results in the vibration of the mesh 144. The operation of thecontrol sub-system component 20 ₁ is discussed below with reference toFIG. 4.

The mesh 144 acts as a gas-liquid interface. The liquid side is composedof the fluid feed system 12 that is configured to bring the sample intocontact with the mesh. The gas side is the collection chamber 16 whichreceives sample droplets D issuing from the mesh.

As shown in FIGS. 1 and 2A, the collection chamber 16 consists of anouter tube 160 and an inner tube 162 defining the chamber 16 betweenthem. A gas feed line 165 from subsystem 20 ₂ provides a carrier gase.g. argon, to the chamber through a gas inlet 164. Preferably, the gasshould be under relatively low pressure, i.e. under 100 psi. The outertube 160 thus provides a path for the low pressure carrier gas to flowto the gas interface of the mesh 144, or right to left in the directionsof the arrows in FIG. 1. The outer tube directs the flow around a cornerat the proximal end 161 of the inner tube 162 and the gas blends withthe sample droplets D produced by the vibrating mesh 144 in spray head142. The blended carrier gas and droplets then flow left to right as anaerosol A inside the inner tube 162 toward a source interface 18 leadinginto the atomizer (not shown) of the atomic spectrometer 100. In theembodiment depicted in FIG. 2A, a torch adapter 166 is integrated intothe interior of the inner tube 162, to more precisely direct the aerosolA toward and into the source interface 18 at the distal end of theadapter 166. As best seen in that figure, the torch adaptor defines anaxial passage 168 which may have a flared or funnel-shaped proximal end168 a that captures the droplets D and carrier gas and guides theblended aerosol A through the elongated body of the torch adapter 166.The shaped proximal end 168 a of the passage essentially compensates forslight mis-alignments between the mesh 144 and the adaptor 166. Theadaptor passage may instead have a proximal end 168 a that isessentially the same diameter as passage 168, or is one with slightlygreater or lesser flare than the funnel-shaped end depicted in FIG. 2A.

FIG. 3 depicts the control sub-system component 20 ₁ in more detail. Adrive waveform generator 220 operates in a known manner to produce aperiodic drive waveform 400 (FIG. 4). The waveform duty cycle may becontrolled by a user, by means of, for example, software control ofgenerator 220. A DC power supply 240 provides power and relatively highvoltage to the drive waveform generator 220, the two constituting adriver for the transducer. Example drive waveforms are depicted in FIG.4. As shown there, the drive waveform 400 ₁ is a square wave,illustrated as having a frequency of 75 kHz. The frequency of the drivewaveform is selected to correspond to the resonant frequency of thespray head transducer 148 (FIG. 2B). The drive waveform 400 ₁ has a dutycycle of 20% and thus drives the transducer, and, in turn, the vibrationof the mesh 144 for 20% of a 1 kHz modulation envelope. The drivewaveform 400 ₂ has a duty cycle of 40% of the 1 kHz modulation envelope.The duty cycle is selected to control the total volume of the sampledroplets that comprise the liquid aerosol A.

Referring to FIGS. 2B and 4, a transition 401 in the square wave 400activates the transducer, which, in turn, causes the mesh 144 tovibrate, and droplets D of the sample are produced by the respectivenozzles 146. When, for example, the mesh vibrates in the direction ofthe cavity 124, the nozzles pick up liquid from the cavity, and when themesh then vibrates in the opposite direction, the nozzles eject theliquid out of the opposite side as droplets D. Accordingly, a knownquantity of droplets in a uniform droplet distribution is produced inresponse to each signal transition in the drive waveform. The mesh 144,shown in enlarged formats in FIG. 5, is a thin steel sheet withlaser-drilled holes of a selected diameter that form the nozzles 146. Anexample of the nozzle size is 2 microns, which produces droplets in anarrow range of sizes smaller than 10 microns. The nozzles in thedesired configuration are separated by a selected distance, for example,70 microns. The selected size, number and separation of the nozzles inthe mesh 144 results in a desired burst quantity of monodispersedroplets D. By controlling the duty cycle of the drive waveform, a usercan precisely control the volume of the sample by controlling the rateof drop bursting in the aerosol A. This is in contrast to theconventional pneumatic nebulizer, which does not allow for such uservolume control.

Referring now to FIG. 6, another embodiment of the sample introductionsystem 10 utilizing PAG is shown for an atomic spectrometer 100 (FIG.1). As before, the fluid feed system 12 includes a fluid feed cavity 124and a fluid feed inlet 122 that leads to the fluid cavity 124 which, inturn, holds a liquid against the liquid interface of the spray head 142.The fluid outlet 126 from the fluid cavity is not visible in theorientation of this drawing. A carrier gas, such as argon, is introducedto the outer tube 160 through a gas inlet, which is also not visible inthe orientation of this drawing. The carrier gas flows along chamber 16into contact with droplets produced by the spray head 142, and a blendof the droplets and the carrier gas is directed as an aerosol along theaxial passage 168 of the torch adaptor 166 into and through the sourceinterface 18 to the remainder of the system shown in FIG. 1. Theproximal end 168 a of the adapter passage 168 is much more graduallyflared than the flared end of the adapter 166 in FIG. 2A. Also, tube 162is joined to the elongated body 169 of the adapter to simplifyconstruction and allow for better alignment of the liquid and gas sidesof the apparatus.

As discussed above, the control sub-system component 20 ₂ of the sampleintroduction system 10 provides user control of the carrier gas flowrate independently of is the aerosol generation controlled by sub-system20 ₁. Unlike the pneumatic nebulizer, the present sample introductionsystem does not require a high pressure gas stream. Accordingly, a user,through sub-system 20 ₂, can readily control of the gas flow rate of therelatively low pressure carrier gas by controlling an associated valveor pump (not shown), manually or through software control.

Independently controlling aerosol generation and carrier gas flow rateproduces a higher quality aerosol and greater transport efficiency,which results in increases in detection limits. Furthermore, the precisecontrol of aerosol generation provides the ability to producemulti-point calibration curves from fewer or even a single standard. Inaddition, the precise control results in less sample production andoptimized consumption such that sample waste is reduced overall. Inaddition, the control may lead to a reduction in the amount of samplebeing transported into an injector (not separately shown) of the atomicspectrometer 100 at times when the optical sensing of signal is notoccurring but when a matrix-laden solution is present at the liquidinterface of the mesh 144. This may, in turn, result in reduced needfor, or frequency of, system maintenance.

High transport efficiency (>80%), uniform droplet size and consistentflow rate were observed when the improved sample introduction system,employing the PAG and providing sample volume control and/or independentgas flow rate control, was utilized for ICP optical emissionspectrometry (OES). These attributes result in demonstrated improvementsin the analytical figures of merit for ICP OES. By comparison, knownsample nebulizers used in ICP OES produce aerosols either pneumaticallyor with an ultrasonic transducer, both of which are less efficient andless precise because they produce poly-disperse aerosols which requireuse of a spray is chamber to remove the largest fraction of droplets,thereby generating a large proportion of waste. A significant fractionof droplets are further lost in transport, with transport efficienciesof 1-5 percent being common for pneumatic nebulizer/spray chamberarrangements.

It is to be understood that the foregoing illustrative embodiments havebeen provided merely for the purpose of explanation and are in no way tobe construed as limiting of the invention. Words used herein are wordsof description and illustration, rather than words of limitation. Inaddition, the advantages and objectives described herein may not berealized by each and every embodiment practicing the present invention.Further, although the invention has been described herein with referenceto particular structures, materials and/or embodiments, the invention isnot intended to be limited to the particulars disclosed herein. Rather,the invention extends to all functionally equivalent structures, methodsand uses, such as are within the scope of the appended claims. Thoseskilled in the art, having the benefit of the teachings of thisspecification, may affect numerous modifications thereto and changes maybe made without departing from the scope and spirit of the invention.

1. A sample introduction system for atomic spectrometry comprising aspray head including a mesh having opposite faces, a sample flow pathfor conducting a liquid sample to one face of the mesh and a transducerfor vibrating the mesh; a driver for driving the transducer to expelsample droplets from the other face of the mesh; an elongated tubularadapter defining a flow passage having a proximal end aligned with andspaced from the mesh and a distal end; a tubular wall surrounding aproximal end segment of the adapter to define a chamber in fluidcommunication with said other face of the mesh and with said proximalend of the flow passage; a carrier gas source connected to the chamber;a flow control device for controlling the flow of gas from the carriergas source to the chamber so that the gas can mix with any dropletsexpelled from the mesh to form an is aerosol in the flow passage, and acontroller for controlling the driver to provide in said aerosol aselected total volume of monodisperse droplets and for independentlycontrolling said flow control device to provide a selected rate of flowof the aerosol along said passage thereby to optimize consumption of thesample.
 2. The system defined in claim 1 wherein the proximal end ofsaid flow passage has a funnel shape.
 3. The system defined in claim 2wherein the funnel shape extends axially beyond the chamber.
 4. Thesystem defined in claim 1 wherein the carrier gas is argon.
 5. Thesystem defined in claim 1 wherein the driver comprises a DC powersupply, and a waveform generator connected between the power supply andthe transducer, said controller controlling the generator to deliver tothe transducer a periodic voltage waveform having a frequencycorresponding to the resonant frequency of the transducer.
 6. The systemdefined in claim 5 wherein the controller controls the duty cycle of thevoltage waveform.
 7. The system defined in claim 5 wherein the waveformis a sine wave or a square wave.
 8. The system defined in claim 1wherein the mesh comprises a thin sheet containing a multiplicity ofperforations having a selected size and a selected separation.
 9. Thesystem defined in claim 8 wherein the sheet is of an acid-resistantmetal or plastic material.
 10. The system defined in claim 1 whereinsaid sample flow path includes a housing defining a cavity facing saidone face of the mesh; an inlet into the cavity, said inlet beingconnected to a sample source; and an outlet from the cavity, said inletand outlet being spaced apart so that the liquid sample is directedacross said one face of the mesh.
 11. A method of introducing a sampleinto an atomic spectrometer, said method comprising the steps ofproviding a spray head including a vibratable mesh; conducting a liquidsample to one face of the mesh; vibrating the mesh to expel sampledroplets from the other face of the mesh into the proximal end of a flowpassage axially spaced from the mesh; conducting a low pressure gas intothe proximal end of the flow passage to mix with the droplets to form anaerosol in the flow passage; controlling the vibrating to provide in theaerosol a selected total volume of monodisperse droplets, andindependently controlling the flow of the carrier gas to providing aselected rate of flow of said aerosol along the flow passage thereby tooptimize consumption of the sample.
 12. The method defined in claim 11wherein the vibrating step is carried out by a transducer contacting themesh, and the vibrating control step is accomplished controlling theduty cycle of a voltage waveform applied to the transducer.